The present invention relates generally to high density routing of circuit traces on a printed wiring assembly (PWA) in a computer system and more particularly to reducing electromagnetic interference (EMI) by providing crosstalk protection between the traces.
One of the greatest challenges of designing PWAs for Electro-Magnetic Compliance (EMC) is providing a means of isolation between noisy components and the rest of the board. Noisy components contaminate the multi-layer substrate with unwanted high frequency noise through a mechanism known as crosstalk coupling, which must be controlled in order to achieve compliance. The most popular method of providing this isolation is through a technique known as "moat & bridge" whereby a channel (moat) surrounding the component of interest is formed by removing the copper from the board surface around the component except for a small area known as the bridge. This absence of copper is mirrored on all layers of the PWA. At the entrance to the bridge, defined by terminal ends of the moat, ground vias are placed on both sides in order to draw high frequency currents to the ground plane prior to their entrance into or out of the protected area.
One of the design parameters that must be followed in order for the moat & bridge technique to work is that all signals to and from the device must cross over the bridge only. Additionally, no other signals may cross the moat on any other layer of the board. Running signals across the moat defeats its purpose and can even cause worse emission problems than not moating at all. Therefore, this technique is only feasible when a small number (typically less than 20) of circuit traces must interface the noise producing component such as in the case of a clock generator.
A problem occurs when very large scale integrated circuits (VLSIC) are noisy. VLSICs often have over one hundred circuit traces that must interface the device. The moat & bridge technique is not a practical option in this situation because there are too many traces to fit in the small bridge area, and a moat is too restrictive from a routing standpoint because the traces fan out in all directions around the component. Unfortunately, a suitable solution to the problems associated with unwanted high frequency noise has not been satisfactorily addressed by the prior art.
Therefore, what is needed is an apparatus and a method for the high density routing of circuit traces whereby crosstalk protection is provided to reduce electro-magnetic interference.